en. xxxii. MALUS ON POLARIZATION. 309 



violet waves are a different length, they will not have met as 

 friends, but as foes, one up and the other doivn, and will 

 destroy each other ; and so will the waves of all the other 

 colours, because they are not of the same length as the red 

 waves. Therefore the only impression on our eye will be 

 that of red. But the bubble is always growing gradually 

 thicker down its sides because the soapy liquid is creeping 

 downwards. So a little lower down the red waves from the 

 two surfaces a and b will no longer fit each other, but will 

 meet unevenly and the red colour will be destroyed. It will 

 now be the turn of the violet rays to combine and make a 

 strong wave to our eye ; a little lower down it will be the 

 turn of the green waves, then of the yellow, and then the 

 film will be thick enough for the red waves to come together 

 again, and so it will go on ; each colour in its turn will pro- 

 duce a strong wave, while all the others are quenched, until 

 the film is too thick for the effect to be produced. 



This is a very rough idea of the way in which the Undu- 

 latory Theory explains the colours which we see in shadows 

 and in the soap-bubble. When you study the subject of 

 light you will see how very complicated these wave move- 

 ments really are ; but without special knowledge you cannot 

 understand more than I have given you here. The colours 

 on mother-of-pearl, on a duck's neck, on the transparent 

 wings of insects, and even on the scum floating on a pond, 

 are all produced by the interference of light, and we owe 

 the discovery of this simple and beautiful explanation to 

 Dr. Thomas Young. 



Mains discovers the Polarization of Light by Reflection, 

 1808. The next step in the science of light was made by 

 Etienne Louis Mains, a young French engineer officer, who 

 was born in 1775, and died in 1812, when he was only 



